David Reath, a plastic surgeon, examines Natalie Robinson, 24, of Knoxville, Tenn. She had plastic surgery for her arms. / American Society of Plastic Surgeons

by Liz Szabo, USA TODAY

by Liz Szabo, USA TODAY

"Arm lifts" have become one of the fastest-growing varieties of plastic surgery, a new study shows.

More than 15,000 women underwent an arm lift in 2012, an increase of more than 4,000% since 2000, according to a report out Monday from the American Society of Plastic Surgeons.

The popularity of the procedures isn't driven by a desire to look as good as Michelle Obama does in a sleeveless sheath.

Instead, the surgeries are growing because more people are losing massive amounts of weight, usually through obesity surgery, says David Reath, a plastic surgeon in Knoxville, Tenn., and chairman of the public education committee for the plastic surgeon society.

About 200,000 Americans a year undergo some kind of weight-loss procedure, such as gastric bypass, says Jack Fisher, president of the American Society for Aesthetic Plastic Surgery, from Nashville.

While surgery can help people lose weight and fat, it doesn't decrease the amount of skin, Reath says. People who lose 100 pounds or more can be left with a lot of extra skin.

"Once skin is stretched out, it becomes like a broken rubber band," Fisher says. "Even when you lose weight, the skin doesn't go back to its normal shape."

Patients sometimes refer to this extra skin as "bat wings," Reath says. Depending on a patient's weight loss and body type, some also opt for surgery to remove extra skin from the abdomen or all around their mid-section, a procedure called a "body lift."

The extra skin can make it harder for patients to wear clothes -- or enjoy their new look, Reath says.

"When they see themselves in the mirror, they don't see themselves at an appropriate weight," Reath says.

The number of cosmetic surgeries increased by 3% last year, to nearly 1.7 million, according to the American Society for Aesthetic Plastic Surgery. When non-surgical options such as botox injections are included, Americans had more than 10 million cosmetic procedures last year.

Arm lifts aren't for everyone.

The surgeries can cost $5,000 or more out-of-pocket, Fisher says. And they leave patients with a scar that stretches from their elbow to their armpit.

Doctors say women who want to look like the first lady will have to earn their triceps the same way she did: at the gym.